1. Field of the Invention
This invention relates generally to a cooling fluid ventilation system for a fuel cell stack and, more particularly, to a cooling fluid ventilation system for a fuel cell stack, where the ventilation system includes a flame arrester within a coolant reservoir, and a compact multi-functional unit for venting and filling the coolant reservoir.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. A PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode reactant gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the MEA. Cathode reactant gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the MEA. The bipolar plates are made of a conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
It is necessary that a fuel cell operate at an optimum relative humidity and temperature to provide efficient stack operation and durability. The temperature provides the relative humidity within the fuel cells in the stack for a particular stack pressure. Excessive stack temperature above the optimum temperature may damage fuel cell components, reducing the lifetime of the fuel cells. Also, stack temperatures below the optimum temperature reduce the stack performance.
Fuel cell systems employ thermal sub-systems that control the temperature within the fuel cell stack. Particularly, a cooling fluid is pumped through the cooling channels in the bipolar plates in the stack. Typically the cooling fluid is a liquid that inhibits corrosion within the stack, does not freeze in cold environments, and is non-conductive. One example of a suitable cooling fluid is a de-ionized water and glycol mixture. It is necessary that the cooling fluid be non-conductive so that current does not travel across the cooling fluid channels in the stack.
The thermal sub-system includes a coolant reservoir that equalizes the thermal expansion of the cooling fluid in the thermal sub-system, and replenishes the small losses of the cooling fluid that occur during stack operation. If the pressure of the gas within the reservoir exceeds a certain pressure, an over-pressure valve will open and release some of the gas to the atmosphere until the pressure is equalized. The coolant reservoir is typically positioned at the highest location in the thermal sub-system.
Hydrogen is a very thin gas and is difficult to contain within an enclosed environment. Because of this hydrogen typically permeates through stack and plate materials and seals within the fuel cell stack, especially around the plates of the stack. Hydrogen leaks into the cooling fluid channels where it is dissolved in the cooling fluid or is trapped in the cooling fluid as hydrogen bubbles.
The impeller of the pump creates cavitation that produces air bubbles that are trapped in the coolant loop. The system includes a ventilation line that allows the air bubbles to be removed from the coolant loop and enter the reservoir. In addition to the air bubbles, the hydrogen bubbles that are trapped within the cooling fluid are also vented to the reservoir, where they accumulate in an air pocket within the reservoir.
This accumulation of hydrogen and air within the reservoir is a combustible source that could ignite. Generally the reservoir includes movable parts, such as the over-pressure valve, that could cause a spark and ignite the combustible mixture of hydrogen and air. Also, the accumulation of hydrogen and air within the reservoir creates a pressure build up therein. The reservoir typically includes a cap that covers a fill port through which the reservoir is filled when necessary. If the cap is removed before the pressure in the reservoir is reduced, the cap may fly upwards under pressure. Thus, it is desirable to remove the pressure within the reservoir before the cap is removed.
Even if the pressure is released in the reservoir before the cap is removed, some ignitable gas may reach the environment. However, if the pressure is reduced, the amount of gas will be so small that it mixes with the ambient air fast enough so that it is not combustible. In other words, if the cap is removed shortly after the fuel cell is operated before the pressure in the reservoir is reduced, and ignitable gas is present in the reservoir, the gas may be released into the environment where it could ignite. If the cap is removed shortly after the fuel cell is operated, but after the pressure in the reservoir is reduced, any ignitable gas present in the reservoir escapes from the reservoir and will mix with the ambient air fast enough so that the danger of ignition does not exist.
To prevent the accumulation of a combustible gas within the air pocket in the reservoir, it is known to periodically remove the hydrogen and gas mixture. Particularly, it is known to pump air into the air pocket in the reservoir, where the existing air/hydrogen mixture within the air pocket is vented from the reservoir through an outlet pipe. This operation removes the hydrogen from the reservoir, while maintaining the air pocket. However, by continually pumping air into the reservoir, the cooling fluid flow from the reservoir may become contaminated with dirt and the like, and therefore, a filter is required. Also, the air from the pump causes some of the cooling fluid to evaporate, which requires that the reservoir be filled more often. Further, the pump may have to be heated so that it does not freeze in sub-zero environments. Also, the ventilation system requires complex electrical systems to guarantee the operation of the ventilation system for safety purposes.
Another possible solution is to include a catalyst in the air pocket that converts the hydrogen and oxygen into water. However, catalysts will operate in this environment only if they are heated to a relatively high degree, where water droplets hitting the catalyst at the surface will immediately evaporate. Also, a device could be added that provides additional air to the reservoir in the event there is a shortage of oxygen in the air pocket to convert all of the hydrogen.